Understanding Media

Part 2 of a 3 part Series

Mike McNamee

Mike McNamee

The tone of the paper is usually defined as warm or cool, bright or natural. Papers described as 'natural' are usually free of OBAs and slightly creamy. Less expensive papers tend to rely on optical brighteners to bring up their base whiteness. A cooler, bright paper is almost certain to contain OBAs. These can be effective for more modern, avant garde images, for fashion images, street portraiture and some types of nature work. The warm- toned papers are favoured by landscape workers, black and white specialists and for some portraits. As ever with photography there will be exceptions to each of these notions.

Coating Technologies

Coating is a relatively new area for things such as inkjet printing and this is where the trouble begins! An inkjet relies on squirting a very tiny drop of ink onto the page. By tiny we mean as low as 3 pico litres. This is a very small drop indeed, you would need 34 million of them to fill a thimble. Even at the 30,000 drops per second this would take the printer well over an hour to fill the thimble!

In terms of spatial resolution, a modern Epson spits 2880 drops in an inch-run of image. The last thing that they want is for this ink to run about all over the surface thus destroying their carefully arranged dot pattern. You have to prevent ink droplets bouncing across the surface and you have to prevent the ink running straight down the microscopically sized holes in the paper surface and disappearing deep into the body of the paper. That is what happens if you print onto a photocopy paper and the result is a dull, lifeless image. If you coat the paper with an impenetrable layer, the ink will sit on the surface, smudge and resolutely refuse to dry. The trick is to take the ink away from the surface but then stop it and hold it close to the surface. Even when you have achieved that aim you are not secure. The ink continues to diffuse into the surface overnight and the print may not look the same colour in the morning. Even if it does, it may still be subject to long-term migration.

As if this is not enough, the different inks diffuse at different rates depending upon their chemical properties which means that there may be a colour shift as the ink dries. A number of different coating technologies are now available to the paper maker

There are a number of paper and paper coating combinations in common use. Most of them make use of the mineral, rutile, which is titanium dioxide, Ti02, a very white powder when it is ground. It should not be confused with titanium golf drivers or hip replacement joints, they use the pure metal, as a cast material. Titanium dioxide is used for all sorts of applications which require whiteness. Your white PVC windows, for example, are a PVC loaded with titanium dioxide. This PVC version is very clever as it consists of little balls of titanium dioxide coated first in a shell of silicon dioxide then an aluminium dioxide shell and then an organic surface coating. Discovered in 1913, titanium dioxide is a ceramic and is frequently mixed with other ceramics and minerals when used as a coating medium. When a coating is referred to as a 'swellable polymer' the outer coating of the ceramic balls is most likely coated with a hydrophilic additive (hydrophilic means water swellable).

For coatings you have a choice of five technologies; Cast Coated, Swellable Polymer, Microporous (meso porous, nanoporous(, Infusion Coated and Bartyta Coatings.

Life Issues

The advantage that swellable polymer had was that the swelling of the coating sealed the surface and reduced the ingress of airborne chemicals (principally ozone and water vapour). This helped to improve fade resistance. Originally it solved an early problem that came to be known as Orange Shift, principally on Epson Premium Glossy Photo Paper, which used a microporous coating. The effect was variable, unpredictable and, at the time (around the year 2000) had a disastrous influence on thinking about inkjet longevity. For a dye ink, swellable coatings last between 2 and 3 times longer, but only from about 8 years to 25 years). Thankfully those days are long-passed and the newer formulations of microporous coatings are very much more stable. Today only a handful of papers use swellable polymer.

The down side of swellable polymer is that the small scale of the surface porosity will sometimes reject the larger particle, pigment materials, flood more easily or not dry at all. Pigment inks always outlive dyes in terms of fade resistance and despite their smaller gamut, higher metamerism and greater manufacturing difficulty they have come to rule the roost that is the quality inkjet market.

For the black and white printer, pigment properties win all round, with the exception of metamerism, and even that is substantially under control with say the Epson K3 UltraChrome ink set. Fade resistance of pigment-based inks, on quality paper for monochrome images, are now quoted out to more than 300 years in ideal conditions and more than 60 years in less protected environments. This white paper, just five years ago would have been talking about 60-70 years for fade resistance in specialised showing environments. The topic has largely fallen off the radar during that time and is no longer a hot topic on forums. However, the warning is clear and producers of fine art, limited edition, high value prints should embrace new technology with caution and seek out only the very best quality materials!